At least 20 people have been killed and scores more injured after a packed metro train derailed between two stations during the morning rush-hour in Moscow.

Emergency crews struggled to reach those trapped in one of the deepest parts of the underground system. As bodies were recovered from the wreckage, the death toll rose.

Reports suggest a power surge may have caused the train to stall, triggering the derailment. Officials say there is no suspicion of a militant attack, the cause of dozens of deaths in Moscow’s underground in years past.

One survivor said his carriage was half-full.

“Suddenly there was a harsh impact and everybody fell down,” he said.

Another man said: “The train came to a sudden halt. Then the electricity was cut and there was smoke everywhere. The carriage fell to one side, off the rails, so people were trapped and could not get out of the train.”

Medical teams and dozens of ambulances rushed to the scene. Helicopters ferried the most seriously hurt to hospital.

The accident happened in the west of the city, between the Slaviansky Boulevard and Park Pobedy stations.

The Moscow metro is the world’s busiest. An investigation is now underway into what is one of its worst-ever disasters, with the city’s mayor declaring Wednesday a day of mourning.